Process for fabrication of large titanium diboride ceramic bodies

ABSTRACT

A process for manufacturing large, fully dense, high purity TiB 2  articles by pressing powders with a sintering aid at relatively low temperatures to reduce grain growth. The process requires stringent temperature and pressure applications in the hot-pressing step to ensure maximum removal of sintering aid and to avoid damage to the fabricated article or the die.

It was developed in research sponsored by Army Materials and Mechanics Research Center, Watertown, Mass. under Interagency Agreement MIPR 83-D071-05, under Union Carbide Corporation contract DE-AC05-840R21400 with the United States Department of Energy.

BACKGROUND OF THE INVENTION

This invention relates to a process for fabrication of large, fully dense, high strength titanium diboride ceramic bodies.

The ceramic titanium diboride (TiB.sub. 2) has numerous exceptional properties including high hardness, high melting temperature, high electrical conductivity, and nonreactivity with various liquid metals that make it an attractive candidate for technological applications such as cutting tools, valve trim for erosive environments, cathodes in Hall-Heroult cells for aluminum smelting and for military armor. In general, the various shapes and sizes of such articles require a capability of manufacturing large, high density, polycrystalline ceramic structures. However densification of TiB₂ ceramics is complicated by two characteristics of this compound, its high melting point and its hexagonal structure. In the first instance, the high melting point of 2980° C. requires sintering temperatures on the order of 2000° C. for pressureless sintering to produce a ceramic having greater than 95% theoretical density; however, maintenance of such temperatures is costly and requires special equipment. Secondly, the hexagonal structure of TiB₂ results in marked thermal expansion anisotropy, i.e., expansion along the c-axis is considerably greater than that along the a-axis. The expansion difference is about 42% between 25° and 930° C., and this difference increases as the temperature exceeds 930° C. The expansion anisotropy produces considerable internal stress during specimen cooling that generates microcracking when the grain size is above a critical value. The microcracking occurs in the grains and at grain boundaries, with a resultant degradation of macroscopic mechanical properties.

At conventional sintering temperatures grain growth readily occurs, so that even if fine grained ceramic powder is used, a relatively large grained ceramic body is produced. To correct these problems a process of hot pressing in a vacuum at a temperature of 1800° C. was developed. However, such high temperatures and large vacuum systems are very costly, die lives are short, and even this lower temperature still promoted unacceptable grain growth.

Therefore, there is a need to provide a method for preparing TiB₂ articles in non-vacuum environments, at relatively low sintering temperatures and with minimum grain growth.

SUMMARY OF THE INVENTION

In view of the need it is an object of this invention to provide fully dense, high strength TiB₂ ceramic bodies in various shapes and sizes.

It is another object of this invention to provide a method of preparing TiB₂ ceramic bodies with minimum grain growth at relatively low sintering temperatures in non-vacuum environments.

Another object of this invention is to minimize strength-limiting microcracks and internal stresses in TiB₂ ceramic bodies upon cooling after sintering.

It is also an object of this invention to provide a process for fabrication of large, dense, strong TiB₂ ceramic bodies that have a minimum of retained sintering aid. Other objects and advantages of the invention will be apparent to those skilled in the art upon study of the specification an appended claims.

The invention is generally described as a process for forming large TiB₂ ceramic bodies by mixing TiB₂ powder with a sintering aid and in a nonoxidizing atmosphere gradually applying and properly maintaining sufficient pressure and temperature to achieve a fully dense, fine grained body. The sintering aid can be nickel or a mixture of nickel and aluminum. The preferred amount of sintering aid is from 1 to 15 wt. percent nickel of a mixture of nickel and aluminum having the nominal composition Ni₃ Al, the preferred amount of the latter also being 1 to 15 wt. percent. Also, the invention is the above described process wherein 0.5 to 2.0 wt. percent carbon is added to the TiB₂ powders and sintering aid. In addition, the invention is a composition of matter of TiB₂ containing from 0.5 to 2.0 wt. percent of carbon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Titanium diboride ceramics have several important characteristics that make them attractive candidate materials for such diverse applications as cutting tools, military armor and cathodes in Hall-Heroult cells for aluminum smelting. In general, such applications require fabrication of TiB₂ in various shapes and with material properties of high density, strength and toughness. Since hot pressing TiB₂ powder in a vacuum atmosphere at a temperature of 1800° C. has many disadvantages, a preferred approach is to use one or more sintering aids that form a liquid film on the TiB₂ grains during sintering. This liquid allows grain rearrangements into a more closely packed structure, and also dissolves some of the TiB₂ to promote rapid diffusive transport of both Ti and B throughout the microstructure. The liquid phase sintering process of this invention enables high density ceramic bodies to be produced at temperatures of 1450° to 1600° C. such that grain growth and the damaging microcracking, characteristic of conventionally fabricated TiB₂, are avoided. It utilizes a nonoxidizing gaseous atmosphere to greatly reduce fabrication costs relative to vacuum processing, and a carefully controlled time-temperature-pressure cycle to promote exudation of the sintering aid as densification progresses. Following this procedure precisely enhances removal of sintering aid and provides high purity TiB₂.

EXAMPLE I

In this process, compacts were uniaxially hot pressed with graphite dies and punches in a press equipped for either vacuum or inert atmosphere. The die cavity was about 2.7 cm ID and was lined with 0.25-mm thick Graphoil, a graphite product made by Union Carbide Corporation, but other suitable compositions capable of absorbing the extruded sintering aid and protecting the die can be substituted. The ends of the punches were also separated from the powder charge by Graphoil. The principal steps in the procedure were as follows:

1. Weigh powders of TiB₂ and a proportionate amount of Ni or other sintering aid to make 1 to 15 wt. percent of the total, typically 38 g.

2. Blend powders about 16 hours in an oblique blender using a blending container having a volume at least three times the volume of the powder.

3. Line die and punch faces with Graphoil and heat in air for about 16 hours at about 110° C. to reduce moisture content.

4. Load powder into cool die in an argon filled enclosure and prepress powder at approximately 0.5 MPa.

5. Install die in press and introduce the desired atmosphere at a pressure of 90 kPa.

6. Heat to about 1550° C. with an applied pressure of approximately 0.6 MPa which is 5 percent of the ultimate pressing pressure of 12 MPa.

7. Increase pressure continuously or in approximately 6 equal increments over a 30 minute period to 12 MPa.

8. Hold at temperature and pressure for 2 hours.

9. After 2 hours release pressure and cool to ambient temperature.

This procedure resulted in compacts measuring approximately 2.7 cm in diameter by 1.2 cm thick.

RESULTS

The hot pressing conditions and properties of compacts prepared in this investigation are shown in the following tables.

SUBSTITUTION OF GASEOUS ATMOSPHERE FOR VACUUM

The compacts of Table 1 revealed the effects of various gaseous atmospheres used during densification on properties of dense compacts. In these experiments the powder blend consisted of TiB₂ and 10 wt. percent Ni. Nitrogen atmosphere yielded poor results. At a pressing temperature of 1450° C., very poor sintering resulted in a density of only 3.08 g/cm³ (the theoretical density of TiB₂ is 4.5 g/cm³), and there was no evidence of Ni removal during pressure. Another compact hot pressed in N₂ at 1550° C. had a density of 5.22 g/cm³, which indicated significant retention of Ni even though some exudation was observed. In addition, microstructural examination indicated that N₂ reacted with TiB₂, possible forming nitrides.

Subsequently, compacts hot pressed in an Ar atmosphere (samples 3, 4, 5, and 6 in Table 1) had desnities near the theoretical density of TiB₂. Microstructoral examination of sample 3 in a plane parallel to the pressing direction revealed a predominantly equiaxed grain structure with a second phase at grain boundaries. An average grain size of 13 μm was measured by the line intercept method.

Single samples were also hot pressed in Ar-4% H₂ and in vacuum. The conclusion deduced from these experiments was that Ar and Ar-4% H₂, but not N₂, are suitable substitutes for vacuum in hot pressing TiB₂. We would also expect that the inert gas helium would be acceptable.

                                      TABLE 1                                      __________________________________________________________________________     Hot pressing conditions and properties of TiB.sub.2                            Principal Variable: Atmosphere Composition, wt %: 90 TiB.sub.2 --10 Ni         Temperature.sup.b : 1550° C. Pressing Conditions: Hot pressing          pressure                                                                       was 12 MPa. Time at pressure and temperature was 2 hours                       except sample 6, which was 6 hours.                                            __________________________________________________________________________     Identity             Sonic Velocity                                                                            Shear                                                                               Young's                                       of TiB.sub.2                                                                               Density                                                                             Linear                                                                              Transverse                                                                           modulus                                                                             modulus                                   Sample                                                                             powder.sup.a                                                                        Atmosphere                                                                            9/cm.sup.3                                                                          km/s km/s  GPa  GPa                                       __________________________________________________________________________     1   C030 N.sub.2                                                                               3.08                                                           2   C030 N.sub.2                                                                               5.22                                                           3   C030 Ar     4.598                                                                               11.09                                                                               7.363 249  552                                       4   B190-1                                                                              Ar     4.459                                                                               10.82                                                                               7.233 233  510                                       5   B190-1                                                                              Ar     4.473                                                                               10.89                                                                               7.371 243  524                                       6   B190-1                                                                              Ar     4.482                                                                               10.89                                                                               7.404 246  526                                       12  B190-1                                                                              Ar--4% H.sub.2                                                                        4.434                                                                               10.504                                                                              7.129 225  484                                       15  B190-2                                                                              Vacuum 4.468                                                                               10.767                                                                              7.318 239  512                                       __________________________________________________________________________                                          Nominal                                   Lame's        Fracture    Fracture   grain                                     Constant Poisson's                                                                           Strength                                                                            Hardness                                                                              Toughness, MPa · m.sup.1/2                                                       size                                      Sample                                                                             GPa  ratio                                                                               MPa  kg/mm.sup.2                                                                           A.sup.c                                                                              B.sup.d                                                                             μm                                     __________________________________________________________________________     2                                                                              3   67.3 0.106                                                                               400 ± 25                                                                         2701 ± 33                                                                          5.1 ± 0.4                                                                         4.4 ± 0.3                                                                        13                                        4   55.4 0.096                                                                               470 ± 80                                                                         2277 ± 95                                                                          6.0 ± 0.6                                                                         8.5 ± 0.9                                                                        6                                         5   44.5 0.077     2207 ± 16                                                                          6.7 ± 1.1                                                                              6                                         6   40.0 0.070                                                                               425 ± 25                                                                          2284 ± 625                                                                        6.4 ± 1.5                                                                         8.2 ± 0.8                                                                        8                                         12  38.5 0.073      2333 ± 325                                                                        7.5 ± 1.1                                                                              6                                         15  39.4 0.071     2300 ± 98                                                                          5.9 ± 0.9                                                                              6                                         __________________________________________________________________________      .sup.a Source  C030, B1901, B1902: Kawecki Berylco Industries, Inc.,           Boyertown, Penn.                                                               .sup.b Sample 1 was at 1450° C.                                         .sup.c Indentation method.                                                     .sup.d Double cantilever beam method.                                    

EFFECT OF Ni CONTENT

The effect of initial Ni content of the blended TiB₂ and Ni powders on compact properties was investigated. The compacts of Table 2, which were arranged in order of increasing initial Ni content, do not show a strong dependence within the range 1 to 10 wt. percent Ni. Sample 9, which had only 1 wt. percent Ni in the starting powder, had the lowest density in the group, while sample 32, which had 3 wt. percent Ni, had the highest density. the densities of samples 8, 9, 10, 11, and 13, each prepared from the same powder, exhibit a small increase with increasing Ni content.

Microstructures of compacts prepared from powder mixtures containing 1, 3, 5 and 10 wt. percent Ni were very similar in terms of size and distribution of the grain boundary phase. Isolated high concentrations of Ni and O were found in the grain boundary phase by electron microprobe analysis. Similar distributions of Ni and O were found in the microstructure in regions near the surface.

The weight loss during hot pressing exceeds the amount of Ni in the initial powder mixture. The difference between predicted and actual weight losses indicates handling losses and/or dissolution of TiB₂ and ejection with the Ni. Loss of some TiB₂ probably occurs, and a small amount of Ni was retained in the grain boundary phase. Most of the Ni exudes from the compacts during hot pressing of powder mixtures initially containing 1 to 10 wt. percent Ni.

                                      TABLE 2                                      __________________________________________________________________________     Hot pressing conditions and properties of TiB.sub.2                            Principal Variable: Ni Content Temperature: 1500° C. Atmosphere:        Ar                                                                             Pressing Conditions: Hot pressing pressure was 12 MPa except                   sample 10 pressed at 24 MPa. Time at pressure was 2 hours.                     __________________________________________________________________________              Initial                                                                        composition of                                                        Identity blended                Shear                                                                               Young's                                   of TiB.sub.2                                                                            powders, wt %                                                                               Sonic Velocity                                                                           modulus                                                                             modulus                                   Sample                                                                             powder.sup.a                                                                        TiB.sub.2                                                                          Ni  Density                                                                             Linear                                                                             Transverse                                                                           GPa  GPa                                       __________________________________________________________________________      9  B190-1                                                                              99   1  4.407                                                                               10.56                                                                              7.129 224  483                                       32  B190-2                                                                              97   3  4.472                                                                               10.450                                                                             7.067 223  482                                        8  B190-1                                                                              95   5  4.413                                                                               10.49                                                                              7.102 223  480                                       10  B190-1                                                                              90  10  4.421                                                                               10.51                                                                              7.144 224  478                                       11  B190-1                                                                              90  10  4.429                                                                               10.417                                                                             7.097 223  476                                       13  B190-1                                                                              90  10  4.428                                                                               10.385                                                                             7.054 220  472                                       __________________________________________________________________________                                          Nominal                                   Lame's        Fracture    Fracture   grain                                     Constant Poisson's                                                                           Strength                                                                             Hardness                                                                             Toughness, MPa · m.sup.1/2                                                       size                                      Sample                                                                             GPa  ratio                                                                               MPa   kg/mm.sup.2                                                                          A.sup.b                                                                              B.sup.c                                                                             μm                                     __________________________________________________________________________      9  43.8 0.082                                                                               393 ± 25                                                                          1883 ± 17                                                                         5.7 ± 0.2                                                                         6.8 ± 0.4                                                                        7                                         32  41.7 0.079      3175 ± 330                                                                        5.8 ± 1.0                                                                              6                                          8  40.5 0.077                                                                               480 ± 15                                                                          2138 ± 3                                                                          5.3 ± 0.1                                                                         6.7 ± 0.1                                                                        8                                         10  36.4 0.070                                                                               525 ± 45                                                                          2120 ± 18                                                                         4.9 ± 0.1                                                                         7.2  8                                         11  34.5 0.067      2152 ± 272                                                                        6.0 ± 0.5                                                                              6                                         13  36.9 0.072            3.4 ± 0.7                                                                              7                                         __________________________________________________________________________      .sup.a Source  B1901, B1902: Kawecki Berylco Industries, Inc., Boyertown,      Penn.                                                                          .sup.b Identation method.                                                      .sup.c Double cantilever beam method.                                    

EFFECT OF TiB₂ POWDER

The compacts of Table 3 were hot pressed under identical conditions with different TiB₂ powders. All powders yielded compacts having densities greater than 98 percent theoretical density except the ART powder. Both compacts pressed with UCC powder, however, contained cracks. The UCC and ART powders had substantially larger average particle sizes than the other powders. These results indicate that the average particle size of the TiB₂ powder should be substantially less than 10 micrometers to ensure highly dense, crack-free compacts.

                                      TABLE 3                                      __________________________________________________________________________     Hot pressing conditions and properties of TiB.sub.2                            Principal Variable: TiB.sub.2 Powder Composition wt %: 90 TiB.sub.2 --10       Ni                                                                             Temperature: 1550° C. Atmosphere: Ar Pressing Conditions: Hot           pressing                                                                       pressure was 12 MPa. Time at pressure and temperature was 2 hours.                                             Lame's                    Nominal              Identity                                                                               Den-                                                                               Sonic velocity                                                                           Shear                                                                               Young's                                                                             Con-           Fracture   grain                Sam-                                                                              of TiB.sub.2                                                                        sity                                                                               Linear                                                                             Transverse                                                                           modulus                                                                             modulus                                                                             stant                                                                              Poisson's                                                                           Hardness                                                                             Toughness, MPa ·                                                      m.sup.1/2  size                 ple                                                                               powder.sup.a                                                                        9/cm.sup.3                                                                         km/s                                                                               km/s  GPa  GPa  GPa ratio                                                                               kg/mm.sup.2                                                                          A.sup.b    μm                __________________________________________________________________________     24 B190-2                                                                              4.462                                                                              10.780                                                                             7.310 238  512  41.7                                                                               0.074                                                                               2610 ± 71                                                                         5.1 ± 0.2                                                                              6                    25 B190-2                                                                              4.458                                                                              10.704                                                                             7.238 234  504  43.7                                                                               0.079                                      38 Starck                                                                              4.460                                                                              10.589                                                                             7.071 223  490  54.1                                                                               0.098                                                                               2100 ± 20                                                                         6.1 ± 0.4                                                                              6                    39 UCC  4.525                                                                              9.793                                                                              6.738 208  433  17.6                                                                               0.039                                                                               1713 ±  20                                                                        5.6 ± 0.3                                                                              7                    41 UCC  4.529                                                                  40 Cotronics                                                                           4.410                                                                              10.174                                                                             6.830 206  448  45.0                                                                               0.089                                                                               2376 ± 30     5                    42 C310 4.417                                                                              10.455                                                                             7.053 220  478  43.4                                                                               0.082                                      43 ART  4.243                                                                              10.420                                                                             6.889 201  448  57.9                                                                               0.112                                      __________________________________________________________________________      .sup.a Sample  B1902, C310: Kawecki Berylco Industries, Inc., Boyertown,       Penn.                                                                          Starck: Hermann C. Starck, West Germany                                        UCC: Union Carbide Corporation                                                 Cotronics: Cotronics Corporation, Brooklyn, N.Y.                               ART: Advanced Refractories Technologies, Inc., Buffalo, N.Y.                   .sup.b Indentation method.                                               

EFFECT OF TEMPERATURE

A group of four samples is arranged in Table 4 to show the effect of hot pressing temperature on properties particularly density in relation to temperature. Nickel (m.p. 1453° C.) is either nearly or actually molten at each pressing temperature in the range 1450° to 1600° C. Apparently Ni aids densification more or less equally at these temperatures. The temperature of 1550° C. used primarily in this work was considered to be sufficient to produce compacts of high density while still retaining the advantage of minimizing detrimental grain growth. There are also obvious economic advantages associated with lower processing temperatures.

                                      TABLE 4                                      __________________________________________________________________________     Hot pressing conditions and properties of TiB.sub.2                            Principal Variable: Temperature Composition wt %: 90 TiB.sub.2 --10 Ni         Atmosphere: Ar Pressing Conditions:                                            Hot pressing pressure was 12 MPa. Time at pressure and temperature was 2       hours.                                                                         __________________________________________________________________________     Identity                       Sonic Velocity  Shear                           Sample                                                                             of TiB.sub.2 powder.sup.a                                                               Temperature (C.°)                                                                 Density 9/cm.sup.3                                                                     Linear km/s                                                                           Transverse km/s                                                                         Modulus GPa                     __________________________________________________________________________      7  B190-1   1450      4.405   10.43  7.026    218                             28  B190-2   1500      4.407   10.453 7.017    217                             33  C310     1550      4.415   10.441 7.035    219                             29  B190-2   1600      4.450   10.470 7.172    229                             __________________________________________________________________________                                        Fracture                                    Young's    Lame's                  Toughness, MPa · m.sup.1/2                                                       Nominal grain                    Sample                                                                             modulus GPa                                                                           Constant GPa                                                                          Poisson's ratio                                                                        Hardness kg/mm.sup.2                                                                    A.sup.b                                                                              B.sup.c                                                                             size μm                       __________________________________________________________________________      7  474    44.5   0.085   2551 ± 26                                                                            5.9 ± 0.4                                                                         2.8 ± 0                                                                          6                                28  473    47.6   0.0899  2314 ± 65                                                                            5.7 ± 0.5                                                                              6                                33  474    44.3   0.0843  3246 ± 304                                                                           5.7 ± 0.8                                                                              5                                29  484    30.0   0.0580  2512 ± 166                                                                           5.4 ± 0.5                                                                              6                                __________________________________________________________________________      .sup.a Source  Kawecki Berylco Industries, Inc., Boyertown, Penn.              .sup.b Indentation method.                                                     .sup.c Double cantilever beam method.                                    

EFFECT OF CARBON CONTENT

Based on observations of modified microstructure in the portion of experimental TiB₂ compacts in contact with the graphite die liners, the deliverate addition of carbon was investigated to determine if (1) the microstructure as modified throughout the compact by addition of carbon and (2) if properties of the compacts were significantly affected. Carbon additions of 0.5 to 2 wt. percent were made as elemental carbon or phenolic resin. Various mixing methods were used including simple dry blending, slurry mixing, ball milling, coating with resin, etc. Resin coating followed by thermal decomposition of the resin to C was less effective because of the high volatility (and resultant loss) of the resin. Ball milling of elementary carbon with the powder was the preferred technique for producing a homogeneous distribution of carbon.

We found that the amount of grain boundary phase decreased with increasing C content and its shape changed from angular to rounded. Both changes would be expected to be beneficial to mechanical properties.

                                      TABLE 5                                      __________________________________________________________________________     Hot pressing conditions and properties of TiB.sub.2                            Principal Variable: C Content Temperature: 1550° C. Atmosphere:         Ar                                                                             Pressing Conditions: Hot pressing pressure was 12 MPa. Time at                 pressure and temperature was 2 hours.                                          __________________________________________________________________________              Initial composition                                                   Identity of blended     Sonic velocity                                         of TiB.sub.2                                                                            powders, wt %                                                                            Density                                                                             Linear                                                                             Transverse                                                                           Shear                                        Sample                                                                             powder.sup.a                                                                        TiB.sub.2                                                                         Ni  C  9/cm.sup.3                                                                          m/s km/s  modulus GPa                                  __________________________________________________________________________     16  B190-2                                                                              89.5                                                                              10  0.5                                                                               4.492                                                                               10.978                                                                             7.417 247                                          17  B190-2                                                                              89.5                                                                              10  0.5                                                                               4.481                                                                               11.090                                                                             7.395 245                                          18  B190-2                                                                              89.5                                                                              10  0.5                                                                               4.492                                                                               10.963                                                                             7.427 248                                          19  B190-2                                                                              89.5                                                                              10  0.5                                                                               4.395                                                                               10.435                                                                             7.071 220                                          20  B190-2                                                                              89 10  1.0                                                                               4.505                                                                               11.118                                                                             7.524 255                                          21  B190-2                                                                              89 10  1.0                                                                               4.500                                                                               11.109                                                                             7.494 253                                          22  B190-2                                                                              89 10  1.0                                                                               4.477                                                                               11.061                                                                             7.403 245                                          23  B190-2                                                                              89 10  1.0                                                                               4.382                                                       26  B190-2                                                                              89 10  1.0                                                                               4.497                                                                               11.033                                                                             7.540 256                                          27  B190-2                                                                              89 10  1.0                                                                               4.490                                                                               11.089                                                                             7.450 249                                          30  B190-2                                                                              94  5  1.0                                                                               4.490                                                                               11.007                                                                             7.492 252                                          31  B190-2                                                                              96   3 1.0                                                                               4.487                                                                               11.113                                                                             7.530 254                                          14  B190-2                                                                              88 10  2.0                                                                               4.556                                                                               9.920                                                                              6.821 213                                          __________________________________________________________________________              Lame's          Fracture   Nominal                                    Young's  Constant                                                                            Poisson's                                                                           Hardness                                                                             Toughness, MPa · m.sup.1/2                                                       grain size                                 Sample                                                                             modulus                                                                             GPa  ratio                                                                               kg/mm.sup.2                                                                          A.sup.b                                                                              B.sup.c                                                                             μm                                      __________________________________________________________________________     16  534  47.1 0.081                                                                               2285 ± 184                                                                        5.9 ± 0.3                                                                         3.2 ± 0.8                                                                        8                                          17  539  61.0 0.0997                                                                              2216 ± 104                                                                        5.8 ± 0.8                                                                              6                                          18  533  44.3 0.0759                                                                              2148 ± 87                                                                         5.3 ± 0.7                                                                              7                                          19  473  39.1 0.0755                                                                              2027 ± 160                                                                        5.8 ± 1.2                                                                              8                                          20  550  46.8 0.0775                                                                              2223 ± 57                                                                         4.5 ± 0.5                                                                         3.5 ± 0.2                                                                        8                                          21  547  49.9 0.0825                                                                              2245 ± 53                                                                         4.4 ± 0.3                                                                         3.6 ± 1.0                                                                        6                                          22  537  57.0 0.0943                                                                              2206 ± 121                                                                        5.2 ± 0.4                                                                              6                                          23                                                                             26  543  36.1 0.0618                                                                              2408 ± 142                                                                        6.0 ± 0.2                                                                              8                                          27  543  53.7 0.0865                                                                              2818 ± 17                                                                         5.7 ± 0.7                                                                              10                                         30  539  39.9 0.0684                                                                              2402 ± 36                                                                         5.8 ± 0.8                                                                              6                                          31  547  45.3 0.0756                                                                              2420 ± 81                                                                         6.0 ± 0.1                                                                              5                                          14  448  24.5 0.052                                                                               1924 ± 154                                                                        5.2 ± 0.5                                                                         10                                              __________________________________________________________________________      .sup.a Source  Kawecki Berylco Industries, Inc., Boyertown, Penn.              .sup.b Indentation method.                                                     .sup.c Double cantilever beam method.                                    

OTHER SINTERING AIDS

The samples of Table 6 were blended and pressed with the compounds CoB, Tab₂, W₂ B₅ and Ni₃ Al as densification aids rather than Ni. Other investigators determined that TiB₂ containing additions of CoB, or CoB plus TaB₂ or W₂ B₅, minimized grain growth and improved mechanical properties of TiB₂ ceramics. Our results show that the compact densified with Ni₃ Al had slightly larger grains but was harder and about as tough as the ceramics densified with CoB or CoB plus TaB₂ or W₂ B₅. the advantage of the Ni₃ Al as densification aid over CoB, TaB₂ or W₂ B₅ is in cost, and based on the hardness values we would predict a stronger ceramic.

                                      TABLE 6                                      __________________________________________________________________________     Hot pressure conditions and properties of TiB.sub.2                            Principal Variable: Hot Pressing Aid Temperature: 1550° C.              Atmosphere: Ar Pressing Conditions: Hot pressing                               pressure was 12 MPa. Time at pressure and temperature was 2                    __________________________________________________________________________     hours.                                                                         Identity     Initial composition of blended powders, wt                                                                   Sonic velocity                      Sample                                                                             of TiB.sub.2 powder.sup.a                                                               TiB.sub.2                                                                             Ni      C.sup.b                                                                               Density 9/cm.sup.3                                                                     km/s                                                                               km/s                            __________________________________________________________________________     34  B190-2   99     1 CoB          4.396   10.557                                                                             7.137                           35  B190-2   94     1 CoB   5 TaB.sub.2                                                                           4.536   10.522                                                                             6.997                           36  B190-2   94     1 CoB   5 W.sub.2 B.sub.5                                                                     4.455   10.579                                                                             6.693                           37  B190-2   91.8   8.2 Ni.sub.3 Al                                                                               4.438   10.948                                                                             7.302                           __________________________________________________________________________                                                   Nominal                                        Young's                   Hardness                                                                             grain size                       Sample                                                                             Shear modulus GPa                                                                        modulus GPa                                                                           Lame's Constant GPa                                                                       Poisson's ratio                                                                        kg/mm.sup.2                                                                          μm                            __________________________________________________________________________     34  224       483    42.1       0.079         6                                35  222       490    58.0       0.104   2100 ± 71                                                                         4                                36  200       466    99.5       0.166   2233 ± 20                                                                         4                                37  237       520    58.7       0.099   2580 ± 92                                                                         10                               __________________________________________________________________________      .sup.a Source  B1902: Kawecki Berylco Industries, Inc., Boyertown, Penn.       .sup.b Except as noted.                                                  

EXAMPLE II Scale-up of Fabrication Procedure

Having demonstrated that a gaseous atmosphere (Ar or Ar-H₂) could be substituted for vacuum during hot pressing of TiB₂ with Ni (or Ni₃ Al) as a densification aid, the next goal was to fabricate nominal 15 cm×15 cm×2.5 and 25×25×5 cm TiB₂ ceramic tiles. Six of the smaller and three of the larger tiles were produced using an argon atmosphere, with duplicate runs made under vacuum. Essentially the same procedure was used as described above except that a pressure of 24 MPa was used (instead of 12 MPa) to ensure high density and low residual Ni content in these large bodies. We found that if this process were rigorously followed that fine-grained, essentially fully dense TiB₂ ceramics were produced, and that the material s were the same for either vacuum or inert-gas processing. The ceramic bodies also had excellent strength (>400 MPa) and fracture toughness (>6 MPa·m^(1/2)). The procedure is specific and slight deviations can result in an unacceptable product or die damage.

For example, increasing the ram pressure over a shorter period of time than the 30 minutes ramp time as specified in this processing procedure caused the failure of very large filament-wound graphite dies apparently as the result of hydraulic forces exerted by the liquid nickel on the side wall of the die cavity. when the ram pressure is slowly applied, the pressure on the side-wall is minimized and the compacting body behaves much like a densifying ceramic with no liquid present.

In another instance two 25.4×25.4×5 cm tiles were densified by hot-pressing in vacuum. In both cases the conditions were identical except for one case the time-temperature-load procedure was deviated, resulting in a product with 7.1 wt. percent retained nickel and a fracture toughness of 5.8 MPa·m^(1/2). In the second case the procedure was followed as previously specified and a ceramic body was produced having only 1.9 wt. percent retained nickel and corresponding improved toughness of 6.7 MPa·m^(1/2).

When five 15×15×2.5 cm tiles were densified by hot-pressing following the process of this invention they had an average retained nickel content of only 1.1 wt. percent indicating the effectiveness of the specific procedures.

These examples demonstrate that the hot pressing process in a nonoxidizing environment under pressure at lower temperatures using a sintering aid results in a large TiB₂ article with small grain size, free of cracks and with good strength. The economy of this process makes it attractive for industrial fabrication of large TiB₂ ceramic bodies. 

We claim:
 1. A process for forming TiB₂ ceramic bodies having a flexural strength greater than 400 MPa and a density greater than 99% of theoretical density comprising:forming a mixture comprising TiB₂ powder and a sintering aid selected from the group nickel, nickel aluminide and mixtures thereof; lining a die with a no dense nonreactive material capable of absorbing said sintering aid and of protecting said die from interacting with said TiB₂ powder; in a nonoxidizing atmosphere placing said mixture in said die; in a nonoxidizing atmosphere applying pressure of about 5 percent of final pressure and raising temperature to a level sufficiently low to avoid grain growth an said TiB₂ but sufficiently high to liquify said sintering aid under final pressing pressure said temperature being about 1550 C.; increasing pressure at a sufficiently slow and steady rate so that sintering aid liquefaction, material transfer, temperature dispersing and compaction occur at a suitable rate to maximize strength, toughness and density of a resulting TiB₂ ceramic and to cause said sintering aid to migrate to the surface of said ceramic body said rate being from 5 percent of final pressure pressure over about 30 minutes, the final pressure being from 12 to 24 MPa; in a nonoxidizing atmosphere, holding said final temperature and said final pressure stable for not less than two hours; releasing said pressure and lowering said temperature to ambient conditions.
 2. The process of claim 1 wherein said non dense, nonreactive material is graphite.
 3. The process of claim 2 wherein said sintering aid is 1 to 15 wt. percent of said mixture.
 4. The process of claim 3 wherein said mixture contains 0.5 to 2.0 wt. percent carbon 